int l[] = {
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 0, 0, 0, 0, 0, 0, 0,
  0, 2, 4, 6, 8, 1, 3, 5, 7, 9 };

int
c_luhn2( char *s ) {
    return ((
        l[ (int)s[ 0] ]
      +    s[ 1]
      + l[ (int)s[ 2] ]
      +    s[ 3]
      + l[ (int)s[ 4] ]
      +    s[ 5]
      + l[ (int)s[ 6] ]
      +    s[ 7]
      + l[ (int)s[ 8] ]
      +    s[ 9]
      + l[ (int)s[10] ]
      +    s[11]
      + l[ (int)s[12] ]
      +    s[13]
      + l[ (int)s[14] ] - 7 * '0' ) * 9) % 10;
}